Subordinating conjunctions also join two clauses
together, but in doing so, they make one clause dependent (or "subordinate")
upon the other.

A subordinating conjunction may appear at a
sentence beginning or between two clauses in a sentence.

A subordinate conjunction usually provides a
tighter connection between clauses than a coordinating conjunctions does.

Loose:

It is raining, so we have an
umbrella.

Tight:

Because it is raining, we
have an umbrella.

Punctuation Note:

When the dependent clause is placed first in a
sentence, use a comma between the two clauses.When the independent clause is placed first and
the dependent clause second, do not separate the two clauses with a comma.